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Chapter: Arrays
Program Notes
In the first line of the sketch we declare an array with four elements.
When using this technique to create a new array we must indicate the
datatype of the array and use brackets[] to tell Processing that we are
about to create an array (and not just simply declare a variable). We
must then name the array and use the assignment operator to populate
the array with the new elements (which do not have values assigned to
them as of yet). We do this by use of the new keyword, followed by the
datatype of the new elements which must match the array's datatype and
finally, brackets indicating how many of these new elements will
populate the array.
int[] myArray = new int[4];
Next the variable clicks is declared, this variable will be used to
count how many times the user has clicked in the Display Window. The
draw() function is then initiated in order to keep the program running,
and prevent the sketch from turning to static mode.
The mouseClicked() function consists of a simple if() statement. The
conditional of the if() statement is executed when the user clicks a
mouse button within the Display Window. The conditional checks if the
clicks variable is less than four, which is the amount of elements in
the array. If this is the first time the mouse has been clicked, clicks
will equal a value of 0 as it has not been assigned a value through
initialization, it has only been declared. As a result the following
statement is run:
myArray[clicks] = mouseX;
This is the statement that assigns a value to each element in the
array. If this is the first time clicks has been used in the comparison
it will also be the first time the user has clicked in the Display
Window and therefore clicks will have a value of 0, as a result the
preceding statement can be translated and explicitly expressed as:
myArray[0] = mouseX;
This means the element with the index number 0 is assigned the value of
whatever the mouse's X coordinate was at the time the user clicked a
mouse button. The next four println() statements print the value of
each of the four elements in the array. As only the first element has a
value at this point in time, it will be the only element that is
non-zero until the user clicks again (up to three more times).
clicks++;
In a similar way that we used augmented assignment to shorten an
expression that is commonly used in programming, we have used an
increment operator in the previous example to shorten this statement.
The increment operator works by adding 1 to the current value of the
particular variable it is used with, then assigning this new value back
to the original variable. This is a common method of getting a program
to count for you as each time the variable is used with the increment
operator it will return a value that is one more than it's previous
value. Writing this statement without the increment operator would look
like this:
clicks = clicks + 1;
As you can see using an increment operator can make your code easier to
read and save you some keystrokes.
The decrement operator (which is two minus signs --) works in a similar
way to the increment operator except that it subtracts 1 from the
variable's current value before reassigning the new value back to the
original variable.
The result of this statement is that the clicks variable will now be
one more than it's previous value by the time the program repeats. For
example the clicks variable was initially 0, but at the end of the if()
statement the clicks variable will have a value of 1. This is important
because the next time the user clicks a mouse button the comparison
will be working with different values. For example on the first click
the comparison can be expressed as (0<4), on the second click the
comparison can be expressed as (1<4) and so on. This continues until
after the fourth click, as the fourth click will equate to (3<4). At
the end of the if() statement on the fourth click the variable clicks
is incremented to 4, this means on the fifth click the conditional can
be expressed as (4<4), this is false and the if() statement is no
longer executed.
If we did not include the if() statement which serves the main purpose
of counting the amount of clicks, the sketch would still have run but
on the fifth click we would be trying to assign a value to an element
in the array that does not exist, as a result Processing would have
given us an “Array Index Out of Bounds” exception. You cannot add
elements to an array like this, there is however a method of getting
around this restriction, which we will have a look at in our mystery
shape maker sketch.
It's worth noting the print out that appears in the text area every
time the user's mouse is clicked for the first four times within the
Display Window. What is most notable about this, is that the current
position of the users mouse's X position is printed along with all the
other X positions that the user's mouse was in when the user clicked up
to three more times previously in the Display Window. All of these
values are now assigned to elements in an array and we now have the
option to use those values in a more complex program, by accessing them
through their index numbers.
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